Preferential remineralization of phosphorus from organic matter in river-dominated coastal sediments

In coastal sediments characterized by substantial terrestrial input, the Redfield ratio may not be adequate to determine whether phosphorus (P) is preferentially remineralized relative to carbon (C). Employing a two end-member delta C-13 mixing model, we observed a gradual decrease in the fraction of terrestrial organic matter as the distance from the river mouth increased. Consequently, the C/P ratio of sedimentary organic matter before early diagenetic alteration (C-u/P-u) decreased from 213 +/- 26 to 126 +/- 4. In contrast, the C/P ratio of sedimentary organic matter after early diagenetic alteration (C-org/P-org) increased from 208 +/- 32 to 265 +/- 23. The deviation of C-org/P-org ratios from C-u/P-u ratios suggests that P was preferentially remineralized from organic matter relative to C. Moreover, the degree of preferential remineralization (DPR) of P, represented by (C-org/P-org)/(C-u/P-u), increased with the distance from the river mouth, suggesting a connection to cross-shelf transport. Besides preferential P remineralization, the control mechanisms for P regeneration from sediments strongly depend on the dissolved oxygen (DO) levels of bottom water. Under oxygenated bottom water (DO >120 mu M), the precipitation of Fe oxides reduced benthic DIP flux, resulting in a C/P ratio in flux well above the C-u/P-u ratio (1813 +/- 725 vs. 213 +/- 26). Conversely, when bottom water DO was low (DO<100 mu M), the dissolution of Fe oxides and preferential P remineralization increased DIP fluxes, but the precipitation of authigenic apatite suppressed DIP fluxes, leading to C/P ratios in flux approximating C-u/P-u ratios (129 +/- 35 vs. 158 +/- 10 and 200 +/- 82 vs. 141 +/- 7). In a moderate redox state (100 < DO <120 mu M), preferential P remineralization and the dissolution of Fe oxides increased DIP fluxes, resulting in C/P ratios in flux below C-u/P-u ratios (29 +/- 8 vs. 131 +/- 5 and 15 +/- 6 vs. 126 +/- 4).